1. Field of the Invention
The present invention is related to a method of creating super hydrophilic surfaces, super hydrophobic surfaces, and combinations thereof. In addition, the invention is related to creation of such surfaces on consumer products such as, for example, electronic devices, optical devices, and others, to enhance performance of the product.
2. Brief Description of the Background Art
This section describes background subject matter related to the invention, with the purpose of aiding one skilled in the art to better understand the disclosure of the invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art.
Super-hydrophobic and super-hydrophilic materials are typically characterized by reference to a water contact angle with the surface of the material. A water contact angle which is greater than about 120 degrees is typically considered to be indicative of a super hydrophobic material. Some of the more advanced super hydrophobic materials exhibit a water contact angle in the range of about 150 degrees. A super-hydrophilic material is typically characterized by a water contact angle of 0 (zero) degrees, which results in an instantaneous wetting of the surface of such a material.
Super hydrophobic surface properties are very desirable in a number of consumer product applications, as the surface is protected from wetting and contamination. For example, an electronic device which might otherwise be shorted out upon becoming wet may be treated to provide a protective hydrophobic surface which keeps the device clean and dry.
Super hydrophobic surfaces may be created by processing of an existing surface. Typical methods of converting material surfaces to become super hydrophobic include, for example: 1) Etching the existing surface to create specific nano-patterns (patterns which are in the nanometer size range), and subsequently coating the surface with a hydrophobic coating. 2) Roughening the substrate surface using techniques known in the art, and functionalizing the resulting surface by applying a hydrophobic coating. 3) Growing a rough (or porous) film from solutions containing nano-particles or polymers in a way which creates a rough and hydrophobic surface on the material.
Generally hydrophobic surfaces have been created in recent years by deposition of common fluorocarbon coatings over a surface. Such fluorocarbon coatings may be created by application of self-assembled perfluorocarbon monolayers (fluorine-containing SAMs), for example. However such surfaces tend to have a water contact angle which is less than about 120 degrees. To obtain a higher water contact angle, it appears to be necessary to texturize the surface prior to application of such a fluorocarbon coating. Although a substrate modified to contain a nano-pattern alone can provide a super-hydrophobic behavior (if the material is hydrophobic to begin with) with respect to a given liquid, a combination of both the nano-patterned surface with a hydrophobic surface finish is helpful in providing and maintaining long term super-hydrophobic behavior of a surface.
Most known man made materials are either hydrophilic or hydrophobic, with their respective surface wetting properties varying within a wide range. Roughening and texturing of a material surface for the purpose of creating a super-hydrophobic or a super-hydrophilic surface is typically done using one of the following methods, each of which has respective advantages and disadvantages.
1. Micro-nano-patterning by material removal: a) The surface may be chemically etched. For example, XeF2 may be used to pattern etch silicon, or HF may be used to pattern etch a silicon oxide. b) The surface may be patterned by plasma etching, such as that used in photo and nano-imprint lithography. c) The surface may be random patterned (roughened) using ion beams, biased plasmas, or laser ablation. d) The surface may be patterned by directly writing a pattern using an electron beam or a laser, or may be patterned through a mask using plasma etching.
2. Surface Texturing: a) The surface may be thermally embossed or imprinted when the material is a thermoplastic. b) The surface may be laser treated when the material is polymeric. c) Inorganic material surfaces may be high temperature annealed, such as in the high temperature annealing of polysilicon.
3. Material deposition or treatment: a) A liquid coating of colloidal nano-particles or a gel may be spin-coated over the surface of a substrate material. b) A porous surface layer may be created over a material by casting of a polymeric precursor in combination with a non-miscible substance, such as with moisture, for example. c) A metal surface may be treated using micro-arc oxidation.
The effectiveness of coating of a textured or roughened surface to produce a super-hydrophobic behavior is typically limited by adhesion of the super-hydrophobic coating material to the substrate material surface. Therefore, many materials (e.g. plastics, polymers, and certain noble metals) may require the use of adhesion layers such as silica, alumina, or other adhesion promoters which are applied over the substrate material surface prior to application of a super-hydrophobic coating material over a textured or roughened surface. Physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD) and other deposition techniques may be used to apply an adhesion-promoting layer. The surface of an adhesion-promoting layer applied in such a manner is typically smooth and replicates the underlying material surface topography.
Bharat Bhushan et al., in U.S. application Ser. No. 11/094,867, filed Mar. 31, 2005, and titled “Hydrophobic Surface With Geometric Roughness”, describes the texturing of a surface to form asperities structures, with subsequent coating or functionalizing of the geometrically patterned surface to produce hydrophobic properties. The Abstract teaches: “A hydrophobic surface comprising a substrate and a roughened surface structure oriented on the substrate material is provided. The substrate comprises a surface, which is at least partially hydrophobic with a contact angle to liquid of 90 degrees or greater. The roughened surface structure comprises a plurality of asperities arranged in a geometric pattern according to a roughness factor, wherein the roughness factor is characterized by a packing parameter p that equals the fraction of the surface area of the substrate covered by the asperities. The p parameter has a value from between about 0.5 and 1.” An exemplary drawing of such a surface shows hemispherically topped pyramidal asperities.
N. Zhau et al., in an article entitled “Fabrication of Biomimetic Superhydrophobic Coating with a Micro-nano-binary Structure”, Macromolecular Rapid Communication, 205, 26, 1075-1080, describe fabrication of a super-hydrophobic coating by casting a polymer solution of bisphenol A polycarbonate (PC) in the presence of moisture. The method comprises a controlled solvent evaporation from the casting solution in the presence of moisture. A porous polymer surface is formed with a micro-nano-binary structure which is said to be similar to that of a lotus leaf.
The authors teach that they clearly showed from the experimental results that humid air was a crucial element for the formation of the hierarchical structure. In addition, the authors propose that casting a polymer solution in the presence of moisture has frequently been used in the fabrication of porous polymer membranes. The influence of moisture on the morphology of the resulting membrane is said to largely depend on the miscibility of the solvent with water. With a water-immiscible solvent, water micro-droplets will condense on the solution surface due to evaporation cooling and then act as a template. After solidification, a honeycomb-patterned porous film is said to be formed. The paper provides illustrations of SEM images of coatings cast at room temperature at different relative humidity, ranging from 20% up to 75%.
In an article by L. Zhai, et al., titled “Stable Superhydrophobic Coating from Polyelectrolyte Multilayers”, Nanoletter, 2004, V4, 1349-1353, the authors describe the formation of super-hydrophobic surfaces by coating a honeycomb-like surface covered with silica nano-particles. The coating applied over the surface is formed by a chemical vapor deposition (CVD) of tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane (a semifluorinated silane). Superhydrophobicity is said to have been achieved by coating this highly textured multilayer surface with a semifluorinated silane. The super hydrophobic surface is said to maintain its super hydrophobic character even after extended immersion in water.
In one embodiment, the authors describe a layer-by layer process of forming multilayers which fabricate conformal thin films. The layer-by-layer film application is said to be useful for any surface amenable to a water-based adsorption process used to construct the polyelectrolyte multilayers. In particular, the authors claim to have discovered that, by using an appropriate combination of acidic treatments PAH/PAA 8.5/3.5, films can be induced to form pores on the order of 10 microns and a honeycomb-like structure on a surface. PAH is poly(allylamine hudrochloride), and PAA is poly (acrylic acid). The surface roughness of such films may be 400 nm, for example. A dense film is created, followed by staged low pH treatments, followed by crosslinking at 180° C. for 2 hours. Subsequently, an SiO2 nanoparticle deposition is carried out, in which 50 nm SiO2 nanoparticles are deposited by alternating dipping of the substrates into an aqueous suspension of negatively charged nanoparticles and an aqueous PAH solution, followed by a final dipping of the substrate into the nanoparticle suspension. The surface is then modified by the CVD coating deposition described above. Finally, the coated substrate is treated using a 2 hour baking at 180° C. to remove unreacted semifluorinated silane.
With respect to the production of super hydrophobic surfaces on useful articles, there are nearly unlimited potential applications. One area of particularly beneficial use is to reduce the contamination and corrosion of electronic boards of the kind used in common electronic products. Many failures of electronic consumer products are due to corrosion and electric shorts caused by accidental wetting or atmospheric moisture condensation on components and wiring leads of electronic boards. In addition, spill accidents are statistically responsible for about 20% of all portable electronics replacements. Marine electronics products and products exposed to wet or humid conditions are particularly vulnerable to such failures. Electronic board reliability in humid and wet environment is also critical to performance of consumer electronics portable devices such as GPS, cell phones, PDA assistants, computers, digital cameras, video games and others.
Due to improvements in materials and electronic device packaging technologies, individual device (chip) reliability has reached new levels, above 99.95%. However, the performance of electronic boards, board interconnects and mounting remain as critical bottlenecks for product reliability. Surfaces of most electronic board materials and components are hydrophilic, which promotes moisture condensation and wetting. Therefore, device performance and reliability can be compromised when electronic boards are exposed to liquids or excessive moisture during everyday use. Environmental contaminants which form ionic solutions in a wet environment can result in leakage or shorts between the device leads. Corrosion over time can further damage electrical connections and render devices non-operational. Encapsulation of the entire electronic board with a moisture protective coating can prevent such damage. However, due to a relatively high cost of such protective coatings, and other disadvantages such as, for example, poor heat dissipation, only specialty and military use electronics use board level protection in the form of a moisture-resistant coating. Parylene, silicone, epoxy, urethane, and other similar coatings have been used in the past, each of them having a number of disadvantages and limitations.
Metal oxides, and in particular aluminum oxide (alumina) and titanium oxide (titania) coatings are known to provide moisture protection and are of particular interest as replacements for the kinds of coatings such as the urethane, epoxy, silicone and parylene coatings mentioned above. The metal oxide coatings can be deposited by means such as physical vapor deposition (PVD) or atomic layer deposition (ALD) methods. However, the coatings generated are not super hydrophobic in nature.
Featherby et al., in U.S. Pat. No. 6,963,125, issued Nov. 8, 2005 and entitled “Electronic Device Packaging”, describe an encapsulation method for electronic packaging. The encapsulation is provided by a coating consisting of two layers: 1) an inorganic layer preventing moisture intake, and 2) an outside organic layer protecting the inorganic layer. Both layers are said to be integrated with an electronic device plastic package. Several inorganic materials, such a silicon nitride, aluminum nitride, titanium nitride and other oxides are suggested for formation of the inorganic layer. These layers may be deposited by PVD, CVD, or ALD, to provide a first continuous layer over a substrate. Subsequently, an organic layer, said to be preferably Parylene C (Col. 10) is applied directly over the inorganic layer. The organic layer has the primary function of protecting the brittle inorganic coating during manufacturing steps such as injection molding.
The use of a dual layer of an inorganic alumina film in combination with an overlying alkylaminosilane hydrophobic coating attached to alumina hydroxyl groups was proposed for wear and stiction protection in micro-electro-mechanical (MEMS) devices in U.S. patent application Ser. No. 10/910,525 filed by George et al. on Aug. 2, 2004. The application, is entitled “Al2O3 Atomic Layer Deposition to Enhance the Deposition of Hydrophobic or Hydrophilic Coatings on Micro-electromechanical Devices”. In addition, George et al. proposed the use of ALD alumina films as moisture and gas barriers on polymer substrate surfaces in U.S. application Ser. No. 10/482,627, filed on Jul. 16, 2002, and entitled “Method of Depositing an Inorganic Film on an Organic Polymer”.
With the development of electroluminescent devices, flat panel displays, organic light emitting diodes (OLEDS), and flexible electronics, there is even a stronger need to protect such devices from performance degradation due to oxidation and moisture corrosion. PVD and ALD alumina films have been tried extensively for such application. However, the single or dual layer protective coatings were found to be inadequate in many cases. Subsequently, various multilayer film laminates have been explored and are currently being considered as a hermetic glass package replacement for use in devices requiring a high degree of protection, for example OLEDs. Haskal et al., in U.S. Pat. No. 5,952,778, issued Sep. 14, 1999, and entitled “Encapsulated Organic Light Emitting Device”, proposed an encapsulation scheme to prevent the oxidation and degradation of OLED devices due to exposure to oxygen, water, and other contaminants. The protective covering comprises three contiguous layers, which include 1) a first layer of thin passivation metal; 2) a second layer of thin film deposited dielectric material such as silicon dioxide or silicon nitride; and, 3) a third layer of a hydrophobic polymer.
Park et al., in U.S. Pat. No. 6,926,572, issued Aug. 9, 2005, entitled “Flat Panel Display Device and Method of Forming Passivation Film in the Flat Panel Display Device” teach a low temperature protective barrier film which is a combination of multiple organic and inorganic films. Simultaneous vapor deposition of silicon tetrachloride and dimethyldichlorosilane onto a glass substrate is said to result in a hydrophobic coating comprised of cross-linked polydimethylsiloxane which may then be capped with a fluoroalkylsilane (to provide hydrophobicity). The substrate is said to be glass or a silicon oxide anchor layer deposited on a surface prior to deposition of the cross-linked polydimethylsiloxane. The substrates are cleaned thoroughly and rinsed prior to being placed in the reaction chamber.
Some methods useful in applying layers and coatings to substrates have been briefly described above. There are numerous other patents and publications which relate to the deposition of functional coatings on substrates, but which appear to be more distantly related to the present invention. To provide a monolayer or a few layers of a continuous functional coating on a substrate surface so that the surface will exhibit particular functional properties it is necessary to tailor the coating precisely. Without precise control of the deposition process, the coating may lack thickness uniformity and surface coverage. The presence of non-uniformities may result in functional discontinuities and defects on the coated substrate surface which are unacceptable for the intended application of the coated substrate.
U.S. patent application Ser. No. 10/759,857 of the present applicants describes one kind of processing apparatus which can provide specifically controlled, accurate delivery of precise quantities of reactants to a processing chamber, as a means of improving control over a CVD coating deposition process. The subject matter of the '857 application is hereby incorporated by reference in its entirety.